I've always been very open with nudity and factual when it comes to answering questions about genitals and gender differences. I don't walk around naked, but we shower together and they come into the bathroom when I'm using it. I used to worry about when I should stop showering with my DS, but that just sort of gradually phased out as we added more children and as he got older. We still shower together occasionally, but its not often. I think if it becomes uncomfortable for either parent or child, it should stop. I know different people have different levels of modesty. For me, I want my sons to have a realistic perspective of a woman's body. And I don't want them to be ashamed of their bodies.
Now that I have a daughter though, I'm a bit more protective over her showering with DH. I don't know why it's different...